Why do two different metals in contact corrode faster?

When dissimilar metals are electrically connected in an electrolyte, the more active metal becomes the anode and dissolves preferentially while the nobler metal acts as the cathode, accelerating galvanic corrosion of the active metal.

What does a Pourbaix diagram tell you?

It shows, as a function of potential and pH, whether a metal is thermodynamically stable (immune), expected to dissolve (corrosion), or covered by a protective oxide (passivation), though it indicates tendency rather than rate.

Aqueous Corrosion

Aqueous corrosion is the electrochemical degradation of metals in contact with water-containing environments, driven by coupled anodic dissolution and cathodic reduction reactions.

Definition

The electrochemical oxidation of a metal in a water-containing environment, in which metal dissolution at anodic sites is balanced by a reduction reaction at cathodic sites on the same or a coupled surface.

Scope

This topic covers the electrochemical mechanism of corrosion in aqueous media: the anodic dissolution of metal and the cathodic reduction of oxygen or hydrogen ions, the mixed corrosion potential and corrosion current, thermodynamic stability mapped by Pourbaix diagrams, and the principal forms of attack such as uniform, galvanic, pitting, and crevice corrosion. It is the basis for predicting and quantifying material degradation.

Core questions

What anodic and cathodic reactions combine to corrode a metal in water?
How does the corrosion potential and corrosion current determine the rate of metal loss?
How do Pourbaix diagrams predict whether a metal corrodes, passivates, or is immune?
What distinguishes uniform corrosion from localized forms such as pitting and galvanic attack?

Key theories

Mixed-potential theory: A corroding metal reaches a steady corrosion potential where total anodic and cathodic currents balance; the magnitude of this balanced current, obtainable by Tafel extrapolation, gives the corrosion rate.
Potential–pH (Pourbaix) diagrams: Maps of electrode potential against pH delineate regions where a metal is thermodynamically immune, actively corroding, or passivated, guiding the assessment of corrosion susceptibility.

Clinical relevance

Aqueous corrosion underlies the failure and maintenance of pipelines, bridges, ships, and biomedical implants; understanding its electrochemistry enables life prediction, material selection, and protection strategies that mitigate vast economic and safety costs.

History

Evans established the electrochemical nature of corrosion in the early 20th century; Wagner and Traud formalized mixed-potential theory in 1938, and Pourbaix compiled potential–pH equilibrium diagrams in the mid-20th century, together giving corrosion a quantitative electrochemical foundation.

Key figures

Ulick R. Evans
Marcel Pourbaix
Carl Wagner
Wilhelm Traud

Seminal works

jones1996
pourbaix1974
bard2001

Frequently asked questions

Why do two different metals in contact corrode faster?: When dissimilar metals are electrically connected in an electrolyte, the more active metal becomes the anode and dissolves preferentially while the nobler metal acts as the cathode, accelerating galvanic corrosion of the active metal.
What does a Pourbaix diagram tell you?: It shows, as a function of potential and pH, whether a metal is thermodynamically stable (immune), expected to dissolve (corrosion), or covered by a protective oxide (passivation), though it indicates tendency rather than rate.